Issue:
Since the UBC Zone 4 seismic map was replaced by IBC High Seismic Area map there is confusion. Customers call out "Zone 4 and don't reference NEBS, UBC or IBC.

Product Line:
NetShelter SX, NetShelter VX

Environment:
All versions and serial ranges.

Cause:
Seismic requests for "Zone 4" that don't specify UBC, IBC or NEBS cause confusion. NEBS still used the Zone 4 seismic map, but NEBS is used almost exclusively by Telecommunications companies.
UBC used the seismic Zone map, but UBC has been replaced by IBC which uses a seismic Area map.
It's very likely that some of the non-specific "Zone 4" requests are IBC High Seismic Area requests, simply using the old UBC "Zone 4" terminology. If the request doesn't specify NEBS, IBC or UBC we don't know if we can use SX cabinets, or if we need to use VX cabinets.

Resolution:

• NEBS GR-63-CORE Zone 4 requests require NetShelter VX Seismic cabinets. NEBS is almost exclusively used by Telecommunications companies, and uses the Seismic Zone map.
• Zone 4 UBC or IBC requests can use NetShelter SX cabinets. IBC is a building code which replaced UBC. IBC uses a Seismic Area map.
• The Seismic Zones map and Seismic Area map are generated using different criteria so no correlation can be drawn between the two maps, or seismic standards (Zone or Areas) derived by these maps.

If the "Zone 4" request is not specific it should be questioned. If no further information is available the VX series can be assumed to be the correct choice. The SX product line is significantly larger than the VX product line in both sizes available and accessories available. Determining the correct seismic standard may be advantageous to the customer.

So what's the difference?

USGS (US Geological Survey) has written a very good FAQ that explains the difference, and why Zone based standards are no longer used.
The complete document can be viewed at this link: https://www.usgs.gov/faqs/what-a-seismic-zone-or-seismic-hazard-zone-where-can-i-find-information-seismic-zones-0-1-2-3?qt-news_science_products=7#qt-news_science_products 
Here is the body of that FAQ:

"Building code maps using numbered zones, 0, 1, 2, 3, 4, are practically obsolete. 1969 was the last year such a map was put out by this staff. The 1997 Uniform Building Code (UBC) (published in California) is the only building code that still uses such zones. Generally, over the past two decades, building codes have replaced maps having numbered zones with maps showing contours of design ground motion. These maps in turn have been derived from probabilistic ground motion maps. Probabilistic ground motion maps have been included in the seismic provisions of the most recent U.S. model building codes, such as the new "International Building code," and in national standards such as "Minimum Design Loads for Buildings and Other Structures," prepared by the American Society of Civil Engineers.
 
Zone maps numbered 0, 1, 2, 3, etc., are no longer used for several reasons.
 
(1) A single map cannot properly display hazard for all probabilities or for all types of buildings. Probabilities: For very small probabilities of exceedance, probabilistic ground motion hazard maps show less contrast from one part of the country to another than do maps for large probabilities of exceedance. Buildings: Short stiff buildings are more vulnerable to close moderate-magnitude events than are tall, flexible buildings. The latter, in turn, are more vulnerable to distant large-magnitude events than are short, stiff buildings. Thus, the contrast in hazard for short buildings from one part of the country to another will be different from the contrast in hazard for tall buildings.
 
(2) Building codes adapt zone boundaries in order to accommodate the desire for individual states to provide greater safety, less contrast from one part of the state to another, or to tailor zones more closely to natural tectonic features. Because of these zone boundary changes, the zones do not have a deeper seismological meaning and render the maps meaningless for applications other than building codes. An example of such tailoring is given by the evolution of the UBC since its adaptation of a pair of 1976 contour maps. First, the UBC took one of those two maps and converted it into zones. Then, through the years, the UBC has allowed revision of zone boundaries by petition from various western states, e.g., elimination of zone 2 in central California, removal of zone 1 in eastern Washington and Oregon, addition of a zone 3 in western Washington and Oregon, addition of a zone 2 in southern Arizona, and trimming of a zone in central Idaho.
 
Older (1994, 1997) versions of the UBC code may be available at a local or university library. A redrafted version of the UBC 1994 map can be found as one of the illustrations in a paper on the relationship between USGS maps and building code maps."